Based on the statistics provided by the American Cancer Society, approximately four million people have died from cancer since 1990, and cancer, after heart disease, is the second leading cause of death in the United States. Treatments of cancer usually include chemotherapy, radiation, hormones, immunotherapy, and surgery. Chemotherapy remains a preferred treatment, especially in cancer types that are in inoperable or metastatic forms.
Many cytotoxic agents, including antimetabolites, antibiotics, alkylating agents, and mitotic inhibitors, are now available in chemotherapy. These agents usually destroy both normal and tumor cells. It is desirable to develop an antitumor agent that preferentially destroys tumor cells over normal cells.
Due to their pathological conditions, tumor cells differ from normal cells in that their surrounding blood vessels are poorly organized, resulting in inefficient delivery of oxygen to the tumor site. In other words, tumor cells are hypoxic (oxygen deficient). This unique physiology opens the door to the design of cytotoxic agents that are specific for tumor cells.